The instant invention concerns a method of manufacturing casting molds from a molding sand which contains a binder.
Such a method is disclosed in the prior art in German Pat. No. 2,403,199.
The manufacture of sand casting molds by means of mold machines has in the prior art been accomplished by means of jolting, mechanical pressing, a combination of jolting and pressing, multiple pressing, pressing with elastic pressing units with a membrane, blowing the mold sand into the form box with a subsequent mechanical secondary compressing or by means of high pressure interleaving with a subsequent mechanical secondary compression. In addition, the molding sand interleaving in the form boxes by means of vacuum is described in German Pat. No. 1,097,621.
During the jolting, the unpleasant noise is particularly disruptive. The mechanical pressing, even through producing little noise, will usually result in uneven compressions. Multiple pressing machines have the disadvantage of producing an uneven back surface of the mold. The same holds true in molding with an elastic compression unit. The blowing of the molding sand into the mold box, particularly the lateral blowing into the mold box, has the disadvantage of a sand blast blower effect and results in excessive wear of the mold; additionally, there results thereby at leaky points a strong stress on the surrounding area due to the scattering of sand particles and dust. The scattering of sand particles and dust also results during superatmosphere pressure injection of the molding sand into the mold box, even though this method has been recently improved to such an extent that there results a substantially improved and a more even sand compression compared to earlier results. A still further advancement in this improvement could be made by means of mechanical secondary compression. Unfortunately, during superatmospheric pressure injecting of the form box, it is impossible to avoid a noticeable noise stress on the surrounding area.
In many of the above methods, there is additionally noted the disadvantage that generally the greatest hardness is obtained at the backside of the mold, where the mold is not being utilized to such a great extent. While close to the pattern form areas, which are located near the partial plane, where due to the desired high precision of the form contours, the greatest form hardness is desired, have a lower degree of hardness due to the sand friction on the model. Additionally, forms which have been pressed under high pressure show especially an extensive "spring-back resilience effect" which during and after the consequent countersinking is considered to be an interferring factor.
The above-mentioned German Pat. 2,403,199 discloses an superatmospheric pressure injection method, in which the molding sand being injected into a form box or form frame by means of a grate whereby the second opening of the sand box, which faces away from the sand chamber, is sealed by means of a hollow pattern plate, the inside of which on one side being connected with the form box or form frame space by means of evacuation nozzles, and on the other side is connectable to a vacuum source by means of an evacuation pipe, whereby after the injection of the molding sand into the form box or form frame the molding sand which has already been precompressed by means of the injection will be consequently compressed by means of the pressing plate.
The evacuation of the model plate in this prior art method is mainly for the purpose of removing the injection air from the form box or form frame, and to thereby support and optimize the superatmospheric pressure injection process. The prior art method is thus not a pure vacuum injection method in the sense of the instant invention.
Even though this prior art method has proven to be successful, it has the disadvantage that during the injection of sands of an overall satisfying quality for the form, from the top downward, the sand which is located in the sand chamber, which strongly widens in the "venturi widening" tends to harden, which will result in the formation of a crater during the injection in the venturi neck.
The crater formation in the venturi neck also appears during the working of normal sands when the horizontal measurements of the venture-neck exceed a predetermined dimension.
Due to hardening of the sand in the venturi widening section and in the limited horizontal dimensions of the venturi neck, it was not possible to produce very large form sections with this prior art method.
The vacuum injection as described in German Pat. No. 1,097,621 would not only offer the advantage of very limited noise stresses, but, due to its small degree of air inclusion, it would also open up the possibility of reducing the above-mentioned "spring-back resilient effect". In practice, however, the form described in the above-mentioned patent, could not be entirely accepted, and it functions, especially not in the working of molding sands with a high inner binding power and clay-containing sands in a quality according to hand-forming.